Discriminated Types with Interfaces Not Supported in LinqProvider.V3?

flibustier_seas · 2025-03-11T17:28:13.661Z

Hi everyone,

It seems that casting via the IVehicle interface is not supported in LinqProvider.V3.
When attempting to filter based on a field inside a discriminated subtype, the following query fails:

var user = await collection.Find(x => ((Car)x.Vehicle!).Model == "Test").FirstOrDefaultAsync();

This throws the following exception:

MongoDB.Driver.Linq.ExpressionNotSupportedException : Expression not supported: Convert(x.Vehicle, Car)
because conversion to MongoCases.MongoNestedDiscriminatedUnionCases+Car is not supported.

However, this exact code worked fine in LinqProvider.V2. After upgrading to LinqProvider.V3, the query no longer functions as expected.

Additionally, if I replace IVehicle (an interface) with a regular class, everything works fine. Unfortunately, changing IVehicle to a class is not an option for my case.

Is there a way to make this work while keeping IVehicle as an interface?

Any suggestions or workarounds would be greatly appreciated!

Full Reproduction Code:

using System;
using System.Threading.Tasks;
using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Bson.Serialization.Serializers;
using MongoDB.Driver;
using MongoDB.Driver.Linq;
using NUnit.Framework;

namespace MongoCases
{
    [TestFixture]
    public class MongoNestedDiscriminatedUnionCases : Mongo2GoFixture
    {
        [Test]
        public async Task Test()
        {
            BsonSerializer.RegisterSerializer(new ObjectSerializer(_ => true));

            var mongoClientSettings = MongoClientSettings.FromConnectionString(ConnectionString);
            mongoClientSettings.LinqProvider = LinqProvider.V3;

            var mongoClient = new MongoClient(mongoClientSettings);

            var db = mongoClient.GetDatabase(Guid.NewGuid().ToString().Substring(0, 8));

            var collection = db.GetCollection<User>("users");

            await collection.InsertOneAsync(new User());
            await collection.InsertOneAsync(new User { Vehicle = new Car { Model = "Test" } });

            var user = await collection.Find(x => ((Car)x.Vehicle!).Model == "Test").FirstOrDefaultAsync();

            // MongoDB.Driver.Linq.ExpressionNotSupportedException : Expression not supported: Convert(x.Vehicle, Car)
            // because conversion to MongoCases.MongoNestedDiscriminatedUnionCases+Car is not supported.

            Assert.That(user, Is.Not.Null);
        }

        private class User
        {
            [BsonId] public ObjectId Id { get; set; }

            [BsonElement("vehicle"), BsonIgnoreIfNull]
            public IVehicle? Vehicle { get; set; }
        }

        private interface IVehicle
        {
        }

        [BsonDiscriminator("car")]
        private class Car : IVehicle
        {
            [BsonElement("model"), BsonRequired] 
            public string Model { get; set; }
        }

        [BsonDiscriminator("bike")]
        private class Bike : IVehicle
        {
        }
    }
}

alexov_inbox · 2025-03-14T09:15:01.409Z

        BsonClassMap.RegisterClassMap<Post>(map =>
        {
            map.AutoMap();
            map.SetIsRootClass(true);
            map.AddKnownType(typeof(Poll));
            map.AddKnownType(typeof(Article));
        });

Oleksandr_Poliakov · 2025-03-14T17:03:35.895Z

Hi @flibustier_seas ,

I cannot reproduce the problem using the latest code base. Could you please specify which version of MongoDB Driver are you using?

Thanks

flibustier_seas · 2025-04-01T13:18:55.242Z

Hi @flibustier_seas ,

I cannot reproduce the problem using the latest code base. Could you please specify which version of MongoDB Driver are you using?

Thanks

Hi!

 <PackageReference Include="MongoDB.Bson" Version="2.25.0" />
 <PackageReference Include="MongoDB.Driver" Version="2.25.0" />

Let me know if you need any other details!

flibustier_seas · 2025-04-01T13:23:09.269Z

alexov_inbox:

BsonClassMap.RegisterClassMap<Post>(map =>
        {
            map.AutoMap();
            map.SetIsRootClass(true);
            map.AddKnownType(typeof(Poll));
            map.AddKnownType(typeof(Article));
        });

Hi!

Thanks for your suggestion, but unfortunately, it doesn’t work with interfaces:

MongoDB.Bson.BsonSerializationException : Discriminators can only be registered for classes, not for interface MongoCases.MongoNestedDiscriminatedUnionCases+IVehicle.

Oleksandr_Poliakov · 2025-04-15T18:43:28.196Z

Thank you for confirming the version that being in use.
I’ve checked C# Driver release changelogs and as it turned out the exact problem was addressed in 3.2.0 version. Please find more information in the following Jira ticket: https://jira.mongodb.org/browse/CSHARP-4572

It would be a nice idea to bump the dependency. If it’s impossible to bump it for some reasons, you can try the following workaround:

var user = collection.FindSync(x => ((Car)(object)x.Vehicle).Model == "Test").FirstOrDefault();

Casting to object before casting to the Car could help translate your filter. Such trick helps sometimes. At least it makes the test work for me.

Thanks,
Oleksandr.